1. Field of the Invention
The present invention relates to two-phase permanent-magnet (PM) stepping motors.
2. Description of the Related Art
Japanese Patent Laid-Open No. 5-161333 discloses a stepping motor configured to rotate as a result of an interaction between a magnet and yokes that are excited by a coil.
Japanese Patent Laid-Open No. 2006-121829 discloses a lens driving device used in, for example, a camera. This lens driving device is configured to drive a lens in a translatory motion by utilizing an interaction between a spirally magnetized magnet and a yoke extending along the spirally magnetized magnet.
Regarding the stepping motor disclosed in Japanese Patent Laid-Open No. 5-161333, the yokes are manufactured by press-working. Specifically, this press-working process for manufacturing a yoke involves perforating a flat plate to form magnetic-pole teeth in a planar fashion, and then bending the magnetic-pole teeth upward.
However, if the magnetic-pole teeth are to be made longer in the axial direction of the yoke in order to increase the area of the yoke that faces the magnet without changing the outer diameter of the motor, the magnetic-pole teeth formed in a planar fashion in the perforating step would become too long to fit within the flat plate. For this reason, the magnetic-pole teeth would need to be expanded by, for example, drawing during the bending step.
FIGS. 10A and 10B illustrate a yoke manufactured by press-working. Specifically, FIG. 10A includes part A-1 and part A-2 that illustrate the yoke after a perforating step, and FIG. 10B includes part B-1 and part B-2 that illustrate the yoke after a bending step. The magnetic-pole teeth in part A-1 have a height h that is smaller than an inner radius R. Consequently, the magnetic-pole teeth in part B-2 similarly have a height h′ that is smaller than the inner radius R. Although the height h′ can be increased by performing a drawing step, that would increase the manufacturing cost. In addition, performing a drawing step would not increase the height h′ by a significant amount, and therefore, there is a limit to increasing the height h′.
Consequently, with a press-working technique, it is difficult to form magnetic-pole teeth that are long in the axial direction at low cost. In other words, forming such magnetic-pole teeth by press-working is problematic in that it requires high manufacturing cost.
In addition, since two yokes that are excited by a single coil are provided as separate components, the positional relationship between the magnetic-pole teeth of the two yokes can vary depending on the assembly accuracy, which may inhibit the motor from operating stably.
Regarding the lens driving device disclosed in Japanese Patent Laid-Open No. 2006-121829, the magnetic-pole teeth extend spirally along magnetized segments of the magnet and are relatively long. Since these magnetic-pole teeth extend at a predetermined angle, the adjacent magnetic-pole teeth tend to contact each other during the perforating step.
FIGS. 11A and 11B illustrate a yoke manufactured by press-working and having slanted magnetic-pole teeth. Specifically, FIG. 11A includes part A-1 and part A-2 that illustrate the yoke after a perforating step, and FIG. 11B includes part B-1 and part B-2 that illustrate the yoke after a bending step. Referring to part A-1, since the tip ends of the adjacent magnetic-pole teeth can undesirably contact each other depending on the length L and the slanted angle θ of the magnetic-pole teeth, the design flexibility is limited. In addition, it is difficult to perform the bending step while properly maintaining the slanted angle θ.
Due to these reasons, it is difficult to manufacture yokes by employing a press-working technique, and moreover, in order to manufacture yokes that are sufficiently suitable for motors having stable performance, high manufacturing costs are required.
In addition, since the two yokes that are excited by a single coil are provided as separate components, the positional relationship between the magnetic-pole teeth of the two yokes can vary depending on the assembly accuracy, which may inhibit the motor from operating stably.